The long term goal of this project is to obtain direct, quantitative measurements of oxygen (O2) concentration in the vessels of the eye fundus using the new, noninvasive technique of phosphorescence quenching (PQ). Our specific aims are: 1. a) To adapt the imaging PQ (IPQ) instrumentation for the measurement of pO2 in the vessels of the cat fundus. b) to develop the focal PQ (FPQ) for on-line, continuous recording of the pO2, simultaneously with blood flow by laser Doppler flowmetry (LDF). 2. To optimize the accuracy of pO2 measurements with IPQ and FPQ a) in the capillaries of the cat retina by minimizing the influence of the choroid, and b) in the choriocapillaris by minimizing the influence of the retinal vasculature. To establish correlations with pO2s measured simultaneously with PQ and O2-sensitive microelectrodes. 3. To map with IPQ the pO2 in the retina, choroid and ONH vasculature under a) normal conditions; b) in response to various stimuli such as: hyperoxia, mild hypoxia, acute changes in IOP, ischemia and flicker induced changes in retinal neuronal activity. 4. To simultaneously record the time courses of the pO2 with FPQ and blood flow by LDF at discrete sites of the ONH, retina and choroid under the conditions mentioned under 3., in order to better understand the role of O2 in the regulation of blood flow to these tissue. We will also study the spontaneous fluctuations in blood flow and pO2 (vasomotion) and the process of a-v shunting of O2. 5. To optimize PQ, using normal rhesus monkeys (Macaca mulatta), for the application of the technique in animals which spontaneously developed diabetes mellitus (similar to type II diabetes in humans), during a subsequent renewal following this 3-year grant. The studies in cats will help us better understand the role played by O2 in the regulation of retinal, choroidal and ONH blood flow. The measurements in normal rhesus monkeys will allow us to optimize the technique before using it in diabetic animals. Further studies in these animals may reveal hypoxic regions and/or deficit in the delivery of O2 to the retina. PQ may ultimately provide the means to quantitatively assess the potential value of drugs to affect blood flow an oxygenation in diabetes.